During the screening of fiber material, such as fiber suspensions of cellulosic material, the object is generally to separate coarse particles in the form of shives, fiber bundles or other impurities from the fiber material. The screening is usually carried out in a screen with a screening cylinder, which can be stationary or rotary. The screening cylinder is generally provided with holes or slits, through which the fiber, but not the impurities, can pass.
Screening cylinders of this type are usually manufactured by drilling holes or milling slits in a metal sheet, which is then formed into a cylinder. In order to obtain an open area, i.e. that portion of the cylinder surface which is perforated as much as possible, the holes or slits must be arranged in a tight relationship. At the same time, the strength of the cylinder must be maintained. This therefore requires high precision work.
Particularly during the manufacture of slitted cylinders, which in certain cases are more advantageous than cylinders with holes which are drilled, difficulties concerning the strength arise, because the material between the slits will become rather thin. The slits, therefore, must be made short and arranged at a certain relationship from each other. This, in turn, results in a certain restriction of the open area.
One way to solve this problem is to form the cylinder from a great number of bars, which are kept in place by annular strips. This, however, is a complicated and expensive way of manufacturing a screening cylinder. It is, furthermore, difficult to provide the slits with a uniform width across the entire cylinder.